1. Field of Invention
The present invention relates to a process using electromagnetic energy in the radiofrequency region to catalyze selective chemical reactions that remove sulfur and nitrogen oxides from flue gas.
2. Background
Coal is a major energy resource of the United States and must be utilized in increased amounts if energy independence is to be a viable goal. A major problem associated with coal combustion is the resulting emissions of sulfur dioxide (SO.sub.2) and nitrogen oxides (NO.sub.x) into the atmosphere. Current flue gas removal technologies are not only expensive and cumbersome, but also produce troublesome waste products. High volumes of chemicals currently are required for SO.sub.2 removal while NO.sub.x removal often uses expensive platinum catalysts. High conversions remain a difficult goal for these current technologies for the convenient chemical reactions require high activation energies, and thus, high temperatures.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions, since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The detection of the radiated spectrum to determine the energy levels of the specific atoms or molecules is called radiofrequency spectroscopy. Often the so called "fine lines" are of interest, and these are created by the rotational and vibrational modes of the electrons. For instance, refer to L. Stepin, Quantum Radio Frequency Physics, MIT Press, 1965.
In the subject invention, the inverse is of interest, that is the absorption of microwave and RF wavelengths by the energy bands of the atoms or molecules resulting in a heating of the nonplasma material and the excitation of valence electrons. This lowers the activation energy required for desirable chemical reactions. In this sense, RF energy can be thought of as a form of catalysis when applied to chemical reaction rates. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.
The electromagnetic frequency spectrum can be conveniently divided into ultrasonic, microwave, and optical regions. The microwave region runs from 300 Mhz (megahertz) to 300 Ghz (gigahertz) and encompasses frequencies used for much communication equipment. For instance, refer to N. Cook, Microwave Principles and Systems, Prentice-Hall, 1986. A narrow part of this microwave region, 915 to 5000 Mhz, is commonly employed for selective heating purposes. Microwave ovens are a common household item and operate normally using 2450 MHz, which is a good frequency for exciting water molecules. However, this type of microwave heating often goes by the common name "RF Heating" and is actually a misnomer for most actual radiofrequencies lie in the what is now called the ultrasonic region. Yet, this concept of using the symbol RF to indicate a catalytic heating action for chemical reactions, regardless of the actual frequencies employed, continues to be commonly found.
Much energy related research was performed in the decade of the 1970s, and a number of U.S. patents were issued. These include:
______________________________________ No. Inventor Year ______________________________________ 3,502,427 Johswich 1970 3,565,777 Lauer 1971 3,656,441 Grey-1 1972 3,765,153 Grey-2 1973 3,869,362 Machi-1 1975 3,887,683 Abe 1975 3,960,682 Baranova 1976 3,981,815 Taniguchi 1976 3,997,415 Machi-2 1976 4,004,995 Machi-3 1977 4,076,606 Suzuki 1978 4,175,016 Lewis 1979 4,940,405 Kelley 1990 ______________________________________
Referring to the above list, Johswich discloses an acid treated activated carbon, giving a higher porosity, for use in removing sulfur, sulfur oxides and nitrogen oxides from flue gases. Lauer discloses a process to decompose sulfur dioxide by first electrically charging water used for absorption and then exposing to an ultraviolet light catalyst to enhance sulfur formation. Grey-1 discloses a cyclone wall-film wash for flue gas components that is enhanced by an electrostatic corona discharge. Grey-2 discloses equipment for an electrostatic ionizing process within a cyclone system that removes flue gas components. Machi-1 discloses a process for removing SO.sub.2 and NO.sub.x by employing ionizing radiation or ultraviolet light at specific compositions to enhance their decomposition. Abe discloses a process for the removal of nitrogen oxides by injecting ammonia and absorbing on activated charcoal with a vanadium oxide catalyst. Baranova discloses a process for handling waste gas containing sulphurous-acid anhydride using an inorganic manganese salt as catalyst. Taniguchi discloses a process for removing sulfur dioxide and nitrogen dioxides by using ionizing radiation to form a removable aerosol. Machi-2 discloses an improvement over Machi-1 by employing contaminated air as part of the process. Machi-3 discloses an improvement over Machi-1 by employing high dose rate electron beam irradiation. Suzuki discloses a process for decomposing nitrogen dioxide using microwave irradiation. Lewis discloses a radiolytic-chemical process for gas production employing nitrogen oxides to inhibit secondary reactions.
In a later patent Kelley discloses a two stage furnace pulsed combustor where the first combustor forms soot that is employed to reduce SO.sub.2 and NO.sub.x in the second combustor where calcium is added to react with the sulfur.
Some additional comment is needed concerning Suzuki which uses the standard microwave frequency to decompose NO.sub.x in the presence of typical exhaust gas constituents, such as SO.sub.2, CO.sub.2 and others. Additionally Suzuki works with only homogenous decomposition of NO.sub.x where the microwave energy catalyzes the breakdown of N.sub.2 and 2NO allowing other subsequent reactions to occur. Suzuki provides no information on heterogenous reactions.
Microwave heating was employed in other activities. For instance, Wall et.al. retorted shale oil with a standard microwave source in "Retorting Oil Shale by Microwave Power," 183 Advances in Chemistry Series 329, American Chemical Society, 1979.
The present invention removes NO.sub.x and SO.sub.2 from gas stream but employs pyrolytic carbon or char in conjunction with RF microwave heating to catalyze a series of heterogeneous reactions that allow removal of said constituents. The above prior art, although representing interesting background material, employs significantly different concepts.